Life as we know it is possible on Earth because of a natural greenhouse
effect that keeps our planet about 60o F warmer than it otherwise
would
be (Figure 1). Water vapor, carbon dioxide (CO2 ), and other trace gases, such as methane and nitrous oxide, trap solar heat and slow its loss by re-radiation back to space. With industrialization and population growth, greenho
use gas emissions from human activities have consistently increased. These steady additions have begun to tip a delicate balance, significantly increasing the amount of greenhouse gases in the atmosphere, and enhancing their insulating effect.

A wide variety of activities contribute to greenhouse gas emissions. Burning of coal, oil, and natural gas releases about 6 billion tons of carbon into the atmosphere each year worldwide. Burning and logging of forests contributes another 1-2 billion
tons annually by reducing the storage of carbon by trees (Figures 2 and 3). The result is that the atmospheric level of CO2, the most important human-derived greenhouse gas, has increased 30 percent, fro
m
280 to 360 parts per million (ppm) since 1860 (Figure 4). Over the same time period, agricultural and industrial practices have also substantially increased the levels of other potent greenhouse gases -- methane concentrations ha
ve doubled and nitrous oxide levels have risen by about 15 percent. These gases have atmospheric lifetimes ranging from decades to centuries; today's emissions will be affecting the climate well into the 21st century.

The overall emissions of greenhouse gases are growing at about 1 percent per year. For millennia, there has been a clear correlation between CO2 levels and the global temperature record. Fluctuations of CO2 and temperature have roughly mirrored each othe
r
over the last 160,000 years (Figure 5).
The current level of CO2 is already far higher than it has been at any point during this period. If current emissions trends continue over the next century, concentrations will rise to
levels not seen on the planet for 50 million years.

Which countries account for the largest proportions of CO2 emissions? In 1995, 73 percent of the total CO2 emissions from human activities came from the developed countries (Figure 6). The United States is the largest si
ngle
source, accounting for 22 percent of the total, with carbon emissions per person now exceeding 5 tons per year. Over the next few decades, 90 percent of the world's population growth will take place in the developing countries, some of which are also
undergoing rapid economic development. Per capita energy use in the developing countries, which is currently only 1/10 to 1/20 of the U.S. level, will also increase. If current trends continue, the developing countries will account for more than half
of total global CO2 emissions by 2035. China, which is currently the second largest source, is expected to have displaced the United States as the largest emitter by 2015.